1. Field of the Invention
This invention relates to a cyclic data transmission method, particularly for updating a common cyclic memory in decentralized control of both a synchronous transmission of cyclic data and an asynchronous transmission of events.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an example of a station in a control data way for realizing a protocol, written in the paper entitled "New Fiber Transmission Data Interface Standard (The Emergining FDDI Standard)," Telecommunications, Vol. 21; No. 5, May, 1987.
The station shown in FIG. 1 is connected to a line 1 in a line connecting unit 2 and is provided with a media access unit 3 having a token round timer 3a and a token holding timer 3b, a buffer memory 4 having a transmitting buffer memory 4a and a receiving buffer memory 4b, a DMA transfer control unit 5, a cyclic memory 6, and an event transmission control unit 7, and furthermore, an external control device 8 is connected thereto.
FIG. 2 is a diagram showing a token frame 10. Reference symbol 10-SD is a start delimiter, 10-FC is a frame control, 10-DA is a destination address, 10-SA is a source address, 10-FCS is a frame check sequence, and 10-ED is an end delimiter.
Also, FIG. 3 is a diagram showing a data frame 11. Reference symbol 11-SD is a start delimiter, 11-FC is a frame control, 11-DA is a destination address, 11-SA is a source address, 11-INFO is an information area, 11-FCS is a frame check sequence, 11-ED is an end delimiter, and 11-FS is a frame status.
FIG. 4 shows a whole outline diagram of the control data way formed by using the stations. To a loop-like transmission line 12 are connected stations 9a, 9b, and 9c, each of which has the media access unit 3, the cyclic memory 6, and the event transmission control unit 7, and to each of which one connected external control devices 8a, 8b, and 8c as shown in FIG. 1.
In the case where two, that is, high speed and low speed, update periods are realized as update periods for the cyclic data, packet multiplexing has been adopted in the prior art. In other words, as shown in FIG. 5, the high speed cyclic data frame 5b and the low speed cyclic data frame 5d are separated from each other, written into the transmission buffer memory 4a in accordance with the respective update periods, and queued for with transmission.
In the case, the update period of the cyclic data is limited to the maximum value of the token rotation time.
Here, in order to satisfy the requirement of the update period of the high speed cyclic data 5b, the value of the target token rotation time (hereinafer referred to as TTRT) has to be determined and set. It is assumed that, in the worst case, the high speed cyclic data frame was only transmitted in the preceding rotation cycle; of the token, and in the following rotation cycle; of the token, the first station consumed all of the maximum possible asynchronous bands and moreover, all stations had requested to send of the high speed cyclic data frame and the low speed cyclic data frame. Therefore, the time required for one rotation of the token is shown by the following expression; EQU Tbh+Tbl+(TTRT-Tbh-Td)
where Td is the delay time required for the token to circulate the loop-like transmission line 12, Tbh is the update period of the high speed cyclic data 5b, and Tbl is the update period of the low speed cyclic date 5d. The total sum of the transmitting time for each cyclic data frame in the all stations is Tbh for the high speed cyclic data 5b and Tbl for the low speed cyclic data 5d, respectively.
This time has to satisfy the requirement of the update period of the high speed cyclic data. That is: EQU Tbh+Tbl+(TTRT-Tbh-Td)&lt;Tth.
When this inequality is modified, the following inequality is obtained; EQU TTRT&lt;Tth+Td-Tbl.
Accordingly, when Tbl&gt;Td, then TTRT&lt;Tth.
In other words, the token is required to circulate in a time shorter than the period required for the high speed cyclic data, and the ratio of Td to the rotation time of the token is increased and the asynchronous band is reduced.
Incidentally, the word "timeout" means that a timer counts down to zero. The word "reset" means that a timer is preset to an initial value to start timeout counting.
The conventional cyclic data transmission method is constituted as described above, and it has had a problem that since the update period of the high speed cyclic data (Tth) has to be set to a larger value than TTRT, the token is forced to circulate the ring-like transmission line more than the required number of rotations, and consequently, the transmission time of the asynchoronous data is reduced to the amount of the propagation delay time of the ring-like transmission line.